Neutron absorber materials in the form of large-area thin plates of pulverulent boron carbide embedded in a solid matrix and processes for their preparation are known. These absorber plates are particularly useful for the storage of burnt-out fuel elements from nuclear reactor plants.
Plates have been made from plastic-bonded neutron absorber materials consisting of boron carbide and if needed, diluents and a solid, irreversibly hardened phenolic polymer that is formed as a continuous matrix around the boron carbide particles and the diluent particles. The plates were prepared by blending coarse-grained boron carbide powders and diluents if required with a phenolic resin in solid or liquid form, molding the mixture and curing the phenolic resin binder at temperatures of up to 200.degree. C. without coking. Diluents such as silicon carbide, graphite, amorphous carbon, alumina and silica, have been suggested. The diluents are generally used in particle sizes respectively equal to the boron carbide powder used (see EP-A-2227, EP-B-2276, EP-B-2715 and the corresponding U.S. Pat Nos. 4,225,467, 4,287,145, 4,198,322, 4,156,147 amd 4,215,883).
Although the known neutron absorber materials can be produced in the form of plates having a thickness greater than 2 mm, in view of the plastic matrix present, they are not resistant to high temperatures and have a tendency to emit gaseous substances under radiation.
The disadvantages of plastic-bonded neutron absorber materials are not encountered in ceramic-bonded neutron absorber materials that consist of boron carbide and free carbon. In the ceramic-bonded neutron absorber materials, the boron carbide portion can amount to up to 60% by volume. The ceramic-bonded materials are prepared by mixing boron carbide powders of a certain particle size distribution and a B.sub.2 O.sub.3 content of less than about 0.5% by weight and optionally, graphite powder with an organic resin binder and a wetting agent, molding the mixture under pressure at about room temperature, curing the resin binder at temperatures in the range of about 180.degree. C. and then coking the molded plates, with the exclusion of air, at temperatures up to about 1000.degree. C. (see German Pat. No. 2,752,040 and U.S. Pat. No. 4,252,691).
Ceramic-bonded neutron absorber materials with a plate thickness greater than about 5mm have sufficient mechanical strength. The mechanical strength can be increased by the addition of graphite. However, it has been discovered that thinner plates having a thickness of about 2 mm and an edge length greater than about 500 mm, the mechanical strength is no longer sufficient when using pure boron carbide powder alone. by increasing the fine-grain portion of the boron carbide powder, it is possible to increase the flexural strength at room temperature. However, these plates are very susceptible to corrosion. As used herein, "susceptible to corrosion" means that their flexural strength drops by more than 50%, of the original value, after immersion for many hours in boiling water (2000-3000 hours). However, by adding graphite in amounts sufficient to increase the strength, difficulties in processing occur due to the highly oriented nature of this anisotropic material. The use of pressure during the forming of plates prevents aeration, which results, in lamination and cracking. Such plates are not dimensionally stable since they tend to warp during the coking operation.